1. Field of the Invention
The invention relates to a method of manufacturing precast components from pretensioned prestressed concrete, particularly of prestressed concrete sleepers, and to an apparatus for carrying out the method.
2. Description of the Related Art
Prestressed concrete sleepers in the form of precast concrete components are particular mass-produced articles, wherein the most important consideration is to reduce the manufacturing costs, while at least maintaining, but if possible improving, their quality. This can be achieved either by reducing the material used for the sleepers, or by reducing the manufacturing costs, whether in respect to the equipment provided for the manufacture, or in respect to the work hours required.
In a known prestressed concrete sleeper, the reduction of material used for the sleeper itself is of particular importance, wherein the reinforcement of the monolithic sleeper body is composed of individual tensioning elements in the form of steel rods, steel wires or steel strands which intersect in the center of the sleeper in a vertical plane and from there extend radially in a straight line towards the sleeper ends (DE 38 32 504 A1). By using individual straight tensioning elements and because of the radial extension of the tensioning elements from the center of the sleeper, it is possible in this known sleeper to achieve uniform distribution of the end anchorings over the cross-sectional surfaces of the sleeper ends and a good penetration of the sleeper body with prestressing steel, even if the sleeper ends are wide and the central part is slender; this makes it possible to achieve a high prestressing force without requiring additional untensioned reinforcements. Together with the lower bending moments in the slender central part of the sleeper, these measures result in a substantial reduction in the amount of steel required for the prestressed concrete sleeper.
This known prestressed concrete sleeper can be manufactured either with prestressing against the hardened concrete and with post-tensioning, or in the prestressing bed, i.e., with pretensioning. If the sleepers are produced in the prestressing bed in which the individual tensioning wires are anchored at their ends by adhesion bonding, it is possible to avoid the expense for the anchoring members of the individual tensioning elements which have to be tensioned against the hardened concrete. This expense is sometimes considerable and is incurred, for example, in post-tensioned prestressed concrete sleepers.
Pretensioned prestressed concrete sleepers are usually produced in a long prestressing bed. When manufacturing prestressed concrete sleepers in a long prestressing bed, the tensioning wires are initially put in place, the tensioning wires are tensioned against fixed end abutments and are set in concrete in the tensioned condition. Depending on the length of the prestressing bed or sleeper, it is necessary to provide a large number of formworks which must be arranged on the prestressing bed. The tensioning wires are released from their anchorings or are severed between the individual sleepers only after complete hardening of the sleeper body, so that the tensioning force introduced into the tensioning wires as a tensile force is transmitted to the sleepers as a pretensioning force.
Instead of guiding the tensioning wires over the entire length of a prestressing bed and anchoring the wires only at the ends of the prestressing bed, it is also known that the tensioning wires, which respectively embrace one or more sleepers next to each other or one behind the other, are tensioned against so-called steel tensioning frames and are set in concrete in this condition. For this purpose, appropriately designed formworks must be combined with the tensioning frames (DE 39 31 201 C1). Since the tensioning force of the tensioning wires is in this case built up with respect to the tensioning frames, there is the advantage that the formworks can be recovered relatively early by way of immediate demolding, while the prestressing forces have to be transmitted to the sleepers only after complete hardening of the individual sleepers.
In this known method, the tensioning wires are initially cut to length, the wires are then introduced into the tensioning frames and are tensioned relative to the latter by means of a tensioning device integrated into the tensioning frames. These steps of operation necessarily have to be performed at different work stations, so that the work required for these work steps is comparatively substantial.